Thermal switch with cycling delay



March 1957 E. G. FRANKLIN I 2,786,114

THERMAL SWITCH WITH CYCLING DELAY Filed April 29, 1954 2 Sheets-Sheet '1r 1.6/ I W/ ///A//////Z////M w /j*///////////'///Z .iOY/WO /VD G R PMATTORNEY March 19, 1957 E. G. FRANKL'IN THERMAL SWITCH wmx CYCLING DELAYFiled April 29, 1954 2 Sheets-Sheet 2 I I INVENTOR. EQ MO/VD G.FRA/V/Q/N JMR/Zibwm ATTORNEY United States Patent Q THERMAL SWITCH WlTHCYCLING DELAY Edmond G. Franklin, Minneapolis, Minn, assignor to GeneralMills, Inc., a corporation of Delaware Application April 29, 1954,Serial No. 426,537

11 Claims. (Cl. 200138) This invention relates in general to thermalswitches and pertains more particularly to a switch of this characterhaving improved means for delaying the cycling of the contacts forming apart thereof.

One of the attendant disadvantages in prior art switches of this varietystems from the fact that they sometimes cycle too rapidly, for instance,where the thermally responsive means associated with the switchstructure follows too closely the changes in temperature, endeavoring tocontrol the current in a corresponding manner, the thermally responsivemeans may fluctuate in its operation to such an extent that the contactswill be made and broken very quickly reflecting these spasmodictemperature conditions. It is the relatively short time intervalsbetween engagement and disengagement of the switch contacts that isextremely undesirable, particularly in certain types of appliances whichmust come up to specified standards such as those requirementsstipulated by the National Electrical Manufacturers Association. On theother hand, the sensitivity may be of such a fine degree that there iswhat is commonly termed a chattering of the contacts, thereby precludinga clear cut engagement and disengagement of these elements. This resultsin unnecessary heating of the contacts, rendering them quite vulnerableto wear with an accompanying shorter life span than would normally beexpected of them.

in an endeavor to combat the foregoing disadvantages certain steps havebeen taken to correct the situation. However, by and large these stepshave resulted in either too costly a switch structure or switchstructure that is unduly large and cumbersome. Accordingly, the instantinvention has in view the correction of these problems in an inexpensiveand facile manner.

One feature of the instant invention resides in the utilization ofthermally expandable struts through which the current coursing throughthe contacts is directed. These struts upon expansion or contraction areinstrumental in changing the relative displacement of one contact andits associated arm with the other contact and arm. Such action is inresponse to passage or interruption, respectively, of current throughboth the struts and contacts, the cross section of the struts beingsufficiently small in relation to the current to generate enough heat toprovide the expansion. In this connection, it is desideratum of theinvention that the total amount of current passing through the contactsbe directed through the struts. Stated differently, each of the strutscontemplated in the practising of the present invention is in serieswith each other and the cooperable contacts of the switch.

Another object of the invention resides in the provision of a thermalswitch having the foregoing thermally responsive strut elements whichare arranged so as to amplify the expansion and contraction of thesestruts. By so doing, the struts cause a corresponding but greaterrelative displacement of the contact juncture of the struts in adirection perpendicular to the contact arm.

Other objects will be in part obvious, and in part pointed out more indetail hereinafter.

The inventionaccordingly consists in the features of construction,combination of elements and arrangement of parts which will beexemplified in the construction hereafter set forth and the scope of theapplication of which will be indicated in the appended claims.

in the accompanying drawings:

Figure l is an elevational view, mainly in section, showing one form ofthe invention;

Figure 2 is a fragmentary View corresponding to Figure 1 showing adififerent operational position of the switch parts;

Figure 3 is another view similar to Figures 1 and 2, but showing a stilldifferent positioning of the switch parts;

Figure 4 is a graph showing a typical operation of a prior art switchwithout the improvement incorporated thereinto that is envisaged by thepresent invention;

Figure 5 is a graph depicting a typical operation of the switchstructure forming the subject matter of this invention;

Figure 6 is an elevational view of one of the contact arms before beingbent into the configuration it assumes in Figure 1;

Figure 7 is a detail plan view corresponding to Figure 6;

Figure 8 i an elevational view similar to Figure 1, but showing amodified version of the switch construction and Pig-tire 9 is a planview similar to Figure 7 showing a differently configured elementplaying an important role in the embodiment of Figure 8.

Referring in detail to the drawings, the thermal switch exemplified inFigures 1, 2, 3, 6 and 7 is designated in its entirety by the referenceletter A. The switch A comprises a tubular post in having a shoulder orflange 12 at its lower end and a riveted upper end 14. Intermediate itsends 12 and 14 is disposed a plurality of operating members employed ina stacked arrangement to form the switch A. First it will be observedthat a bimetallic element 16 carrying an upstanding insulating stud 18at its free end encircles at its other end the tubular post it A pair ofcontact arms 24 and 22 also are supported in a cantilever manner byreason of the tubular post it). Subjacent the riveted end 14 is arelatively rigid bracket member 24. Between these various members 16,20, 22 and 24 is interposed a plurality of insulating spacers 26suggestively of ceramic material.

Describing the contact arm 25) in greater detail it is to be discernedthat this arm includes a resilient horizontal element bearing thenumeral 23 and carried by this resilient element is a contact 39. At thefree end of the element 28 and integral therewith is a first inclinedstrut 32 and extending toward the tubular post it) is a second orcompanion strut 34 While these struts may be connected directly witheach other in an integral fashion at their neighboring ends, nonethelessit may at times be desirable to utilize a rivet or beating member 36 toresist wear in this region. With this in mind provision is made forsupporting the rivet 36 by forming what can be termed an annular nexus38 having an aperture 3h extending therethrough, the aperture 44 servingto accom modate the shank of the rivet 36.

Proceeding further with the detailed description of the contact arm 20,the strut 34- merges into a terminal strip 42, this strip having aU-shaped bend intermediate its ends at 44. The U-shaped portion rendersthe strip 42 quite flexible, so as not to impose any unwantedrestriction on the movement of the contact arm 2%. The end thereof lyingopposite the strut 34 is anchored into place by means of the tubularpost 10 and its associated elements. Since it is intended that theterminal portion 42 the conductor for introducing current to the shots.1 and 3 5, a suitable layer of insulation 46 such as mica, is sandwicled between the terminal portion merging into t re strut 34 and theunderside of the resilient element 28. in order to anchor the upper endof the strut 34 with respect to the resilient element a pair ofinsulating grommet halves i8 is utilized. These grommet halves orsections are disposed in suitable apertures 27 and t? contained in theresilient element 2 and the terminal portion 42. A rivet 59 extendsvertically through these grommet halves and in this way the abovealluded to anchoring is achieved.

The second contact arm 22 is arranged in a vertically spacedrelationship with the contact arm 2%. This contact arm carries on itsunder surface a contact 52 which is cooperable with the first mentionedcontact 3%. Integral with the contact arm 22 is a terminal portion 54-,this terminal portion being merely a direct continuation of theresilient arm 22. For adjusting the switch A for a preferred operationtemperature, the bracket 2% has mounted thereon an adjusting shaft 56having a threaded portion 58 which is received in a coriplementalaperture at the distal end of the bracket 25. At the lower end of theshaft 56 there is disposed an insulating stud element 69 which bearsagainst the rivet 36 in the exemplified embodiment. However, it will beunderstood, as earlier herein before pointed out, that the rivet 36 maybe dispensed with if desired. At the upper end of the shaft 56 is a knob62 by which the shaft may be rotated to make the appropriate adjustmentto the temperature at which the contacts 36' and 52 will separate ordisengage. A stop element 64 serves as the agency by which the rotationof the shaft 56 is limited. Thus it will be appreciated that byinherently biasing the contact arm in an upward direction, theinsulating stud 6h opposes such an upward bias and by either raising orlowering the stud (by rotation of the knob 62) the upward bias ismoderated or altered.

To permit the stud 6% to abut against the rivet 36 it will be noted thatthe resilient element 28 contains an aperture 66 therein, and thecontact arm 22 contains a similar aperture 65, the stud 6% extendingdownwardly through both of these apertures. Also it will be recognizedthat the apertures are large enough to provide adequate clearance sothat there is no interference with the movement of the contact arms 29and 22. Further apertures 67 and 69 permit the encirclement of theelement 23 and the terminal portion 42 about the tubular post 10.

Inasmuch as it is contemplated that the switch struc' ture A be utilizedin association with an electric heating appliance, the drawingsadditionally show a member 7%) which may be the soleplate of an electriciron. Embedded in the soleplate is a conventional encased heating unit72, the opposite ends of which emerge upwardly and are designated by thenumerals 7 3 and 76. The end 74- is connected to a conductor 73 leadingto one side of a source of power L; the other end '76 is connected by aconductor iii} to the terminal portion 42, and the terminal portion 54is in turn connected to the other side of the source L by reason of aconductor 82. Thus it can be seen that when the contacts 36 and 52 areengaged a con ductive path is completed via the conductor 78, theheating element 72., the conductor the terminal portion 42, the struts3d and 32, a portion of the resilient element 2 9, the contacts and 52and the conductor (52 via the contact arm 22.

Having in mind the construction and arrangement of the principalelements hereinbefore described, it is believed that a com leteunderstanding of my invention may now be had from a description of itsoperation. Assuming then that the contacts 32' and 52 have just beenbrought together, current will ilow over the path mentioned in thepreceding paragraph. By virtue of the particular cross section of thestruts 32 and 34, however, the flow of current is instrumental incausing these struts to expand longitudinally, and due to the fact thattheir opposite ends are anchored with respect to the resilient element23, the joined ends, that is the nexus 38 containing the rivet 36, willmove downwardly with respect to the element 23. Actually though, sincethe ele ment 23 is inherently biased upwardly, the longitudinalexpansion of the struts and 33 i is responsible for permitting thecontact 3% to bear with greater pressure against the contact than itotherwise would at the first instance of engagement. Of course, once thecurrent has begun to pass through the struts 32 and 36 it is only amoment before these struts expand sufficiently to increase the pressuralcontact of the contact 30 against the contact 52.

Continuing with the operational sequence of the thermal switch A, thecontacts 3d and 52 having met causes current to flow through the heatingelement 72 at the same time that the current courses through the strutsand 34. The heating element 72 in turn heats the soleplate 7i), and asthe temperature of the soleplate is gradually raised, more and more heatwill be radiated and/or conducted to the bimetallic strip 16. The heatreceived by the bimetal 36 causes the strip to warp upwardly and thestud abuts the underside of the contact arm 22 as pietured in Figure 2.It is the upward force applied to the contact arm 22 that causesseparation of the contacts 30 and 52, this immediate separation beingshown by the solid line position of the contact arms 26 and 22. Asexplained above, the cross sectional area of the struts 32 and 34 issuch that these struts heat very quickly due to the passage of currenttherethrough. The converse is likewise true upon the cessation ofcurrent, for once the current has been interrupted then the struts cooland contract. The contracted position of the struts has been shown indotted outline in Figure 2. Such a contraction of course, draws theresilient element 28 downwardly and the separation of the contacts 30and 52 is immediately increased when the current flowing through themhas been interrupted. This is so, since the stud is in an abuttingrelationship With the rivet 36, and the contraction of the struts mustdraw the resilient element 28 downwardly, thus increasing the contactspacing as mentioned immediately above.

The solid line position of the switch parts depicted in Figure 3 is thesame as the dotted line position thereof pictured in Figure 2. It isupon reaching the solid line position of Figure 3 that the contacts 30and 52 are once again brought into engagement with each other and suchengagement again causes current to course through the struts 32 and 34,thereby to heat and expand them. The expanded position of the struts isrepresented by the dotted outline shown in Figure 3 which is identicalwith the switch position pictured in Figure 1. Thus from the foregoingdescription it will be seen that a complete operational sequence hasbeen described for the position shown in Figure 1 started with thecontacts just after they had come together and the struts 32 and 34 hadaccordingly been heated and expanded.

While the above given operational description is thought to be entirelyobvious and straightforward, nonetheless reference may now be had, ifdesired, to Figure 4. Here the curve labeled 84 illustrates thetemperature of the soleplate or other controlled device 70 plottedagainst time and it will be apparent that these cycles occur relativelyrapidly with the peaks 86 and the valleys 88 being quite irregular. Insuch a situation the ordinary switch structure producing the graph ofFigure 4 would not be equipped with the readily expandable struts 32 and34 as in the exemplified embodiment. In countradistinction, the curve ofFigure 5 is symbolic of the operation of the device just described inconjunction with Figures 1 to 3 for here the cycling delay mechanismoperates to produce a wider spacing of the cycles in the curve 90 whileat the same time the peaks 92 and depressions or valleys 94 are far moreuniform.

Turning now to a discussion of Figures 8 and 9,-attention is drawn tothe fact that in large measure the parts constituting the switch B herepresented are comparable or equivalent to those described in detail withrespect to Figures 1 to 3. The primary difference in the modifiedversion resides in the formation of the contact arm corresponding infunction to the contact arm 20 of the earlier described embodiment,therefore, the modified contact arm has been given the reference numeral100 and includes a resilient element or portion 102 which is relativelyshort. This element 102 has integrally connected thereto a strut 104 anda companion strut 106, these two struts as in the other embodiment beingarranged at an obtuse angle with respect to each other. The strut 106merges into a contact support 108 carrying a contact 110 whichcorresponds in function to the contact 30.

Bridging the opposite ends of the struts 104 and 106 is a rigid member112 which may be of ceramic material or metal having an oxidized orsilicone varnish surface 114. The purpose of the coating or surface 114is to provide high electrical resistance contact by the member 112 whensuch member is metallic so that current coursing through the resilientelement 102 will be compelled to traverse the struts 104 and 106 toprovide the requisite heating effect thereof. Of course, if the member112 is composed of ceramic material then the ceramic itself is asatisfactory insulator and precludes the flow of current by way of thismember. Since the member 112 serves to anchor the opposite ends of thestruts 104 and 106 relative to each other, suitable rivets 116 and 118may be inserted through the contact arm 100. To provide for suchinsertion, apertures 120 and 122 are formed in the contact arm. As withthe earlier described struts 32 and 34, if desired, the juncture thereofmay be of annular configuration containing a rivet such as the rivet 36.

Inasmuch as the operation of the switch B is identical with that of theswitch A, it is not deemed necessary to describe the functioning againin detail. In this connection, however, it will be appreciated that whenthe contacts 110 and 52 are engaged current will flow through the switchvia the resilient element 102, being directed through the struts 104 and106 on its way to the contact 110. The current, as in the previousembodiment, is instrumental in causing the struts 104 and 106 to expand,since from Figure 9 it can be seen that these struts are of relativelylimited cross section compared to the cross section of the remainder ofthe contact arm 100. Upon cessation of current flow the struts 104 and106 will again contract just as is the situation with respect to thestruts 32 and 34.

As many changes could be made in the above construction and manyapparently widely different embodiments of this invention could be madewithout departing from the scope thereof, it is intended that all mattercontained in the above description or shown in the accompanying drawingsshall be interpreted as illustrative and not in a limiting sense.-

It is also to be understood that the language used in the followingclaims is intended to cover all of the generic and specific features ofthe invention herein described and all statements of the scope of theinvention which, as a matter of language, might be said to falltherebetween.

I claim:

1. A thermal switch comprising a pair of cooperable contacts forcontrolling an electric current, a first temperatture sensitive devicearranged to effect initial separation of said contacts in response toincreasing temperature, a second temperature sensitive device whichprior to saidinitial separation is expanded through heating by currentcontrolled by said contacts but which cools and consequently contractsafter such separation due to the cessation of said current, therebycausing the gap between said contacts to increase as it cools, and meansdirectly engageable with said second temperature device providingrelative adjustment of said contacts for separation at a preferredtemperature of said first temperature device.

2. A thermal switch comprising a pair of cooperable contactsforcontrolling an electric current, first means including a pair ofinclined struts of electrically conductive material and a memberanchoring the opposite ends of said struts in a substantially fixedspaced relationship with each other, one of said contacts beingelectrically connected to one of said struts, second means carrying theother of said contacts, manually adjustable means engageable with saidstruts adjacent their locus of juncture and thermally responsive meansengageable with said second means, said struts having a cross-sectionlimited with respect to the current controlled by the contacts forcausing substantial expansion and contraction of the struts by heatingand cooling in response to the changes in current carried by the strutson engagement and disengagement of the contacts.

3. A thermal switch in accordance with claim 2 in which said member hasan oxidized surface.

4. A thermal switch comprising a pair of cooperable contacts forcontrolling an electric current, first means including a pair ofinclined struts of electrically conductive material and a memberanchoring the opposite ends of said struts in a substantially fixed,spaced relationship With each other, said member carrying one of saidcontacts, second means carrying the other of said contacts, manuallyadjustable means engageable with said strut-s adjacent their locus ofjuncture and thermally responsive means engageable with said secondmeans, said struts having a cross-section limited with respect to thecurrent controlled by the contacts for causing substantial expansion andcontraction of the struts by heating and cooling in response to thechanges in current carried by the struts on engagement and disengagementof the contacts.

5. A thermal switch comprising a first switch arm including a horizontalelement supported at one end and a pair of inclined metallic strutsconnected together at one end, the other end of one of said struts beingcon nected to the other end of said horizontal element and the other endof the other strut being insulatingly anchored to said horizontal memberat a locus intermediate the ends of said horizontal element, a firstcontact carried by said horizontal element, a second switch arm,

a second contact carried by said second arm cooperable with said firstcontact, manually adjustable means engageable with the connected ends ofsaid struts, and thermally responsive means engageable with said secondcontact arm for moving said contacts relative to each other, said strutshaving a cross-section limited with respect to the current controlled bythe contacts for causing substantial expansion and contraction of thestruts by heating and cooling in response to the changes in currentcarried by the struts on engagement and disengagement of the contacts.

6. A thermal switch comprising a first switch arm including an elongatedresilient element supported at one end, a first metallic strut integralwith the other end inclining away from said element, a second metallicstrut joined to the first strut and inclining toward said resilientelement, terminal means connected to said second strut having a flexibleportion, insulating means between said terminal means and said resilientelement disposed intermediate said flexible portion and said secondstrut, means extending through said terminal means, said insulatingmeans and said resilient element for anchoring the neighboring end ofsaid second strut relative to the resilient element, a first contactcarried by said resilient element, a second switch arm, a second contactcarried by said second arm cooperable with said first contact, manuallyadjustable means engageable with the connected ends of said strut, andthermally responsive means engageable with said second switch arm formoving said contacts relative to each other, said struts having across-section limited with respect to the current controlled by thecontacts for causing substantial expansion and contraction of the strutsby heating and cooling in response to the changes in current carried bythe struts on engagement and disengagement of the contacts.

7. A thern a1 switch in accordance with claim 6 in which said terminalmeans is an integral continuation of said second strut and said flexibleportion constitutes 1-. U-shaped bend disposed substantiallyperpendicularly to the axis of said resilient element.

8. A thermal switch comprising a first switch arm including an elongatedresilient element supported at one end, a first metallic strut integralwith the other end inclining away from said element, a second metallicstrut joined to the first strut at an obtuse angle at one end and havingits other end remote from said element, a member of relatively highelectrical resistance extending between the opposite ends of saidstruts, a first contact carried by said first switch arm adjacent saidremote end of said second strut in an electrically conductive relationwith said struts, a second switch arm, a second contact carried by saidsecond arm cooperable with said first contact, manually adjustable meansengageable with the connected ends of said strut, and thermallyresponsive means engageable with said second switch arm for moving saidcontacts relative to each other, said struts having a cross-sectionlimited with respect to the current coritrolled by the contacts forcausing substantial expansion and contraction of the struts by heatingand cooling in response to the changes in current carried by the strutson engagement and disengagement of the contacts.

9. A thermal switch in accordance with claim 8 in which said member is astrip of metal having an oxidized surface.

l0. A thermal switch in accordance with claim 8 in which said member isa strip of metal having a silicone varnish surface.

11. A thermal switch in accordance with claim 8 in which said member isa strip of ceramic material.

References Cited in the file of this patent UNITED STATES PATENTS2,275,917 Newell Mar. 10, 1942 2,329,119 Jacobs Sept. 7, 1943 2,409,420Clark Oct. 15, 1946 2,716,173 Russell Aug. 23, 1955 2,716,174 Franklinet al. Aug. 23, 1955 2,716,175 Franklin Aug. 23, 1955

